Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227, which is incorporated herein by reference in its entirety. However, the present invention may find applicability in any implantable medical device system. For example, the disclosed invention can also be used with a Bion™ implantable stimulator, such as is shown in U.S. Patent Publication 2007/0097719, filed Nov. 3, 2005, which is also incorporated herein by reference in its entirety.
As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible case 30 formed of titanium for example. The case 30 typically holds the circuitry and power source or battery necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 100 is coupled to electrodes 106 via one or more electrode leads (two such leads 102 and 104 are shown), such that the electrodes 106 form an electrode array 110. The electrodes 106 are carried on a flexible body 108, which also houses the individual signal wires 112 and 114 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead 102, labeled E1-E8, and eight electrodes on lead 104, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary.
As shown in FIG. 2, the IPG 100 typically includes an electronic substrate assembly 14 including a printed circuit board (PCB) 16, along with various electronic components 20, such as microprocessors, integrated circuits, and capacitors mounted to the PCB 16. Two coils are generally present in the IPG 100: a telemetry coil 13 used to transmit/receive data to/from an external controller 12 as explained further below; and a charging coil 18 for charging or recharging the IPG's power source or battery 26 using an external charger (not shown). The telemetry coil 13 can be mounted within the header connector 36 of the IPG 100 as shown.
As just noted, an external controller 12, such as a hand-held programmer or a clinician's programmer, is used to send data to and receive data from the IPG 100. For example, the external controller 12 can send programming data to the IPG 100 to dictate the therapy the IPG 100 will provide to the patient. Also, the external controller 12 can act as a receiver of data from the IPG 100, such as various data reporting on the IPG's status. The external controller 12, like the IPG 100, also contains a PCB 70 on which electronic components 72 are placed to control operation of the external controller 12. A user interface 74 similar to that used for a computer, cell phone, or other hand held electronic device, and including touchable buttons and a display for example, allows a patient or clinician to operate the external controller 12.
Wireless data transfer between the IPG 100 and the external controller 12 takes place via inductive coupling. To implement such functionality, both the IPG 100 and the external controller 12 have coils 13 and 17 respectively. Either coil can act as the transmitter or the receiver, thus allowing for two-way communication between the two devices. When data is to be sent from the external controller 12 to the IPG 100 for example, coil 17 is energized with alternating current (AC), which generates a magnetic field 29, which in turn induces a voltage in the IPG's telemetry coil 13. The generated magnetic field 29 is typically modulated using a communication protocol, such as a Frequency Shift Keying (FSK) protocol, which is well known in the art. The power used to energize the coil 17 can come from a battery 76, which like the IPG's battery 26 is preferably rechargeable, but power may also come from plugging the external controller 12 into a wall outlet plug (not shown), etc. The induced voltage in coil 13 can then be demodulated at the IPG 100 back into the telemetered data signals. To improve the magnetic flux density, and hence the efficiency of the data transfer, the IPG's telemetry coil 13 may be wrapped around a ferrite core 13′.
As is well known, inductive transmission of data from coil 17 to coil 13 can occur transcutaneously, i.e., through the patient's tissue 25, making it particular useful in a medical implantable device system. During the transmission of data, the coils 13 and 17 lie in planes that are preferably parallel. Such an orientation between the coils 13 and 17 will generally improve the coupling between them, but deviation from ideal orientations can still result in suitably reliable data transfer.
As mentioned previously, the IPG 100 can communicate various types of status data back to the external controller 12, such as the voltage of the battery 26 (Vbat) and other operational parameters of interest. Typically, such operational parameters are read and stored by the IPG 100 on a schedule, such as every ten minutes or so. Thereafter, the stored data may be communicated back to the external controller 12 at an appropriate opportunity (such as when the external controller 12 requests communication with the IPG 100), or may be queried by the IPG 100 for its own internal use as appropriate. Typically, such data is stored in a non-volatile memory, such as a flash EPROM memory, so that the data is preserved in the event that the IPG 100 looses power, such as when the battery 26 becomes depleted. Transfer of stored data back to the external controller 12 can allow the user or a clinician to interpret the stored data to some useful end, and such analysis may require connection between the external controller 12 and a computer (not shown).
Operational parameter data is also typically stored in the IPG 100 with a time stamp, which comprises the IPG's understanding of the time that particular data was logged. A time stamp usually comprises a numeric (e.g., binary) value determined by the IPG's internal timing circuitry. Such timing circuitry usually operates in conjunction with the IPG's clocking circuitry, and may be based upon a simple clock pulse counter to cite one simple example.
Unfortunately, the time stamp associated with the stored data can be unreliable for at least two reasons. First, the IPG's timing circuitry may be inherently inaccurate, such that the time stamps deviate from true time by an unacceptable margin. Second, and perhaps most significantly, loss of power in the IPG 100 can cause the timing circuitry to reset. For example, suppose the battery 26 in the IPG becomes so depleted that the IPG's timing circuitry can no longer operate. Although the battery 26 can be recharged and the timing circuitry later enabled, the timing circuitry will have lost its timing reference. Thus, after coming up from a reset condition, the IPG 100's timing circuitry would understand the IPG to be operating at “time zero,” and would incorrectly start stamping logged data accordingly.
The inability to properly establish an accurate time basis for logged IPG data hampers the utility of such logged data, because certain logged data may only be useful when judged against an accurate time basis. To cite just one example, it may be important to assess battery voltage as a function of real time, because the rate of any degradation may be an important factor in determining when the IPG's battery 26 needs to be replaced (by explanting the IPG 100 from the patient). Without an accurate time basis, such battery voltage data, and other time-sensitive operational parameter data, may be of limited use.
Accordingly, it would be beneficial to improve the reliability of the time basis for data logged in an implantable medical device such as an IPG 100. This disclosure presents such solutions.